Orthodontics requires a precision wire cutter tool when installing appliances on teeth. The tool must be compact, reasonably lightweight, strong, capable of exact placement under difficult conditions and sharp enough to cut stainless steel wire with a minimal controlling force. Because of the harsh environment provided by sterilization procedures such as autoclave and chemiclave the tool should also be corrosion resistant. While orthodontic wire cutting tools have become quite sophisticated, they have been unable to meet all of these objectives.
A typical orthodontic wire cutting tool is shaped much like a pair of pliers except that the pinching fingers each carry a facing cutter blade that squeezes and cuts a wire strand between them. Experimentation has shown that the cutting blades or tips should have a hardness between 62 and 67 on the Rockwell c (Rc) hardness scale. If too soft, the cutting edge too readily deforms and looses its sharpness. If too hard, the edge becomes brittle and subject to early fracture.
To achieve an economical combination of cutting tip hardness and tool body handle strength, it has become common practice to make the cutter body from a high grade stainless steel such as type 420 or 425M stainless steel. A relatively hard T15 tool steel cobalt chromium alloy formed into a small, frangible planar cutting tip was then heat treated and subsequently brazed to the body of the cutter tool. The tool with hardened tips in place was then rough ground, the frangible tip web was broken and a finishing grind, buffing and polishing was performed. Final precision assembly to attain proper tightness in the pivot action was accomplished and the cutting tips or blades were sharpened with a diamond hone.
While the cutting tool thus manufactured in accordance with a conventional process is a high quality, precision tool, it has two significant disadvantages. The T15 tool steel used for the cutting tips is subject to corrosion from stringent sterilization procedures like autoclave and chemiclave. This corrosion is conventionally controlled by plating the finished cutter first with nickel and then with chromium. Not only does this plating process increase the cost of the cutting tool, but it is not completely effective. As the tool is used the protective plating quickly wears away along the cutting edge and the cutting tips begin to corrode.
In addition, in order to avoid tempering the hardened T15 tool steel tips as they are brazed to the cutter body, a low temperature foil brazing medium must be used to effect the braze. For this purpose a low temperature silver alloy brazing foil is used to enable a brazing temperature of about 1500 degrees F.
While the low temperature brazing foil preserves the hardness of the cutting tips, it is difficult to get the foil to properly wet the bonding surfaces. As a result a failure rate of the braze joint on the order of 2-3% has been found to occur during use. Even higher failure rates have been encountered when cutting heavy gauge wire.